New Evidence for an N = 32 Subshell

Based on self-consistent energy density calculations, Tondeur [1] proposed N = 32 as a new magic number for neutron-rich nuclides. Following the beta decay of 52K, Huck et al. [2] assigned the 2.56 MeV state in 52 20Ca32 a spin and parity of 2+. As compared to the first excited 2+ level in 50Ca, an increase in E(21 ) was observed. Huck et al. [2] suggested the rise at N = 32 was due to the ν2p3/2 subshell closure, indicating that N = 32 was semi-magic. This assertion was consistent with Tondeur’s theoretical prediction [1]. To determine whether E(21 ) peaks at N = 32 for calcium, the E(2 + 1 ) systematics of heavier calcium isotopes would be of value. However, at present these nuclei are difficult to produce with sufficient statistics. Therefore, the systematics of the chromium isotopes were examined. Similar to 52 20Ca32, the first 2 + state of 56 24Cr32 lies higher in energy relative to its N − 2 neighbor, Cr30. Unlike Ca32, the spin and parity assignment of the 2+ level for Cr32 was deduced from the shape of proton angular distribution curves following its production via the (t,p) reaction [3]. A second (t,p) study confirmed the spin-parity assignments for a number of states, including the first excited 2+ state at 1007 keV [4]. To determine whether the first excited 2+ energies continued to rise or peaked at N = 32, it was necessary to measure E(21 ) values beyond N = 32. In this report, the energy of the first excited 2 + state of 58Cr is presented.